The symptoms you describe are consistent with the pivot hubs (114-1955) & control fork (114-1956) being worn. Once these parts have worn, you are using some of the control handle stroke to remove the slop from the system, thus less stroking of the pump, thus less speed. You need to get the parts replaced, then re-set the controls. Without actually having the machine in front of us, we cannot be 100% certain, but we are pretty confident with this diagnosis.